1. Field of the Invention
The present invention relates to components for communication devices. More particularly, this invention is a surface treatment and method for applying a surface treatment to microwave components or other components where secondary electron emission must be kept low. The surface treatment is a coating comprised of yttrium-iron-garnet, which is preferably applied to the inner surface of the component by sputtering.
2. Description of the Related Art
Secondary electron emission is a well known problem occurring in electrical components exposed to high frequency (e.g., microwave) electromagnetic radiation. Under such electromagnetic field exposure, when an incident electron impacts onto the surface of the component, additional electrons (so-called secondary electrons) can be emitted from the surface of the component along with the incident electron. These secondary electrons are then subjected to the same electromagnetic field exposure as the incident electron, and thus may also impact the surface of the component, leading to the emission of additional secondary electrons. Under certain circumstances, the electrons (incident and secondary) may bounce back and forth inside the component, driven by the electromagnetic field. Each time an electron impacts the surface of the component, an additional secondary electron may be emitted. This phenomenon, known as multipaction, may lead to an electromagnetic field driven regeneration or avalanching of electrons that can cause deterioration of the component, modulation distortion, instabilities with catastrophic consequences and failure modes in which destruction of the entire component may occur. Electrical components that are exposed to such high-frequency electromagnetic fields are therefore designed to minimize the effects of secondary electron emission and multipaction, and thus reduce the likelihood of a corresponding avalanche failure of the component.
The term "secondary yield coefficient" is used in this field to describe the ratio of the number of secondary electrons generated for each incident electron impacting on the surface of a component. If the secondary yield coefficient is greater than one, meaning that more than one secondary electron is emitted from the surface of the component for each incident electron, then an avalanche-like increase in the number of electrons may occur.
Presently, communication components are typically designed to control secondary electron emission and multipaction through manipulation of the inner-surface geometry of the component. Designers recognize that for any given operational frequency, there is a range of component geometries and power levels for which secondary electron emission will occur. Knowing this, the inner surfaces of the component can be designed so as to control or prevent secondary electron emission, and thus prevent an avalanche failure of the component.
Such a design practice, however, is difficult from a manufacturing standpoint and places constraints on the design dimensions and sizes of the component. Simple designs are often eliminated in favor of more intricate and complicated designs in order to maintain safety margins against multipaction. These constraints on the design adversely limit the form and fit of components. In some cases, larger and multiple components are often necessary to effectively prevent or control secondary electron emission. These limitations present particular disadvantages for communication components onboard spacecraft payloads, such as communication satellites, where size and weight must be minimized. Furthermore, analyzing such components is difficult, and time-consuming, and still does not guarantee the elimination of multipaction. Expensive testing is often necessary to verify the analysis. And even after initial verification, the component materials may degrade with time and exposure to a condition that may result in component failure.
Prevention of multipaction has also been attempted by insertion of a dielectric material into the component, typically in the predicted flight path of any electrons. Using such a dielectric material, however, results in an increase in the loss of the electrical component, and thus a reduction in the power handling capabilities of the device. Therefore, for a given power level, the use of a dielectric results in a larger and heavier component, which is undesirable in many applications, such as communication satellites.
Alternatively, coatings have been applied to the inner surface of components in an attempt to either interrupt the electron flight path or prevent avalanching. This alternative is often not feasible in high power designs, however, due to dielectric heating of the surface and increase in loss; any macroscopically thin dielectric inserted in the path of secondary electrons will prevent multipaction, but may render the component unusable due to an increase in the loss.
While several well-known materials have secondary yield coefficients below 1 (e.g., carbon soot), it is another matter to find a material that will adhere to the inner surface of a component, survive environmental and outgassing testing, resist oxidation and the resultant increase in yield, and not increase the loss of the component. In addition, some surface treatments previously used, such as alodine (alodining is the chemical application of a protective chromate conversion coating on an aluminum alloy), provide a coating of uneven and uncontrollable thickness that is high in loss. There is also some evidence that alodine may actually fail to prevent secondary electron emission. Alodining also presents environmental disadvantages, since it involves the use of chromic acid, and thus disposal of the spent solution is difficult and expensive. Other coatings such as carbon black, while having a low secondary electron yield, have a limited adherence to the surface of a component. For example, carbon black is not feasible for use in space applications, because it fails to maintain adherence to the surface under conditions of a vacuum.
Therefore, a surface treatment is needed that does not interfere with the low-loss requirements of typical electrical components, while still providing a highly conductive surface. Unfortunately, most highly conductive surfaces have secondary electron yield coefficients above one and are therefore prone to multipaction. Moreover, after exposure to air, any resulting surface oxidation leads to a further increase in the secondary yield factor and a resultant increase in multipaction. An appropriate surface treatment must also resist peeling at high temperatures, may not outgas, and preferably does not consist of hazardous materials or create hazardous byproducts. Thus, the ideal surface treatment not only has a secondary yield coefficient of less than 1, but also is low loss, highly conductive, resistant to oxidation over time, resistant to changes over temperature increases, capable of withstanding high temperatures without peeling, resistant to outgassing, and consists of non-hazardous materials.